Galactomannans are a class of polysaccharides composed of galactose and mannose. In general, thc structure of the polymers consist of a linear chain of 1,4-linked beta D-mannopyranosyl units with single-membered alpha-D-galactopyranosyl units joined by 1,6 linkages. Galactomannans vary in the extent and uniformity of substitution with respect to their galactose moieties. For example, the two sources of galactomannans are guar and locust bean gums. Galactomannan from guar has a galactose:mannose ratio of approximately 1:2. Galactomannan from locust bean gum has a galactose:mannose ratio of approximately 1:4.
There are three major components of a guar seed, i.e., the seed coat, the endosperm, and the germ. The endosperm contains most of the galactomannan of the seed and consists mainly of galactomannan. In addition to the galactomannan, the endosperm of guar seeds may contain protein (5-7% wt) and small amounts of zinc, lead and copper. During preparation of the galactomannan, some small amount of hull material comprising cellulose from the endosperm may be processed with the galactomannan. In its natural state, the weight average molecular weight of guar galactomannan is typically in the millions.
Galactomannan in its natural state, such as from guar seeds, may have a low molecular weight distribution. The polydispersity index is one measure of molecular weight distribution. Galactomannan in its natural state may be depolymerized to generate shorter length galactomannan polymers. These shorter polymers have a lower weight average molecular weight but tend to have a high polydispersity index. Several studies purport to describe methods for making depolymerized polysaccharides, including galactomannan, and the resulting depolymerized compositions [U.S. Pat. No. 3,728,331 (Savage); U.S. Pat. No. 4,874,854 (Colegrove); U.S. Pat. No. 5,708,162 (Hilbig); U.S. Pat. No. 4,753,659 (Bayerlein); EP 030443A; WO 93/15116; Craig, D., et al. (1992) Proceedings of the Thirty-Ninth Annual Southwestern Petroleum Short Course, Southwestern Petroleum Short Course Association, Inc., Apr. 22-23, 1992, Texas Tech University, Lubbock, Texas; Vijayendran and Bone, (1994) Carbohydrate Polymers 4:299-313; Frollini, E. et al., (1995) Carbohydrate Polymers 27:129-135; Ouchi, T., et al., (1997) J.M.S.—Pure Appl. Chem. A34(6):975-989; Tayal, A., et a., (2000) Macromolecules 33:9488-9484].
None of these patents or publications provides a description of a hydroxypropylgalactomannan composition having the polydispersity index, weight average molecular weight (Mw) and viscosity of the hydroxypropylgalactomannans of the present invention. Further, none these publications provides a method for preparing a composition comprising depolymerized galactomarman according to the present application. Nor do those publications describe the particular benefits of using these compositions in treating and/or making fractures in a subterranean formation such as a well.
One object of this invention is to provide an efficient, high-yield, cost effective method for manufacturing a galactomannan composition. Another object of this invention is to provide a method for manufacturing a galactomannan composition that requires less caustic material and reduces the amount of waste generated during manufacture. Yet, another object of this invention is to provide a method for making a composition comprising galactomannan having a certain molecular weight, polydispersity index and viscosity, which is useful, e.g., in treating or making a fracture in a subterranean formation. In one preferred embodiment, the compositions of this invention are useful as a component of a hydraulic fracturing fluid.
One object of this invention is to provide a galactomannan composition that has the combination of advantages of rendering a subterranean formation treatment fluid, such as a fracturing fluid, highly viscous to carry proppant efficaciously yet sufficiently flowable to fill a subterranean fracture. Additionally, an object of this invention is to provide a galactomannan composition that is useful in making an improved fluid for treating a subterranean formation, wherein the fluid has high conductivity or movement through a proppant pack, improved regained formation permeability and lower dynamic fluid loss. Yet another object of this invention is to provide a galactomannan composition that hydrates uniformly and quickly. Further, other objects of this invention include providing a galactomannan composition useful for making an improved subterranean formation treatment fluid that does not require the use of breakers.
Other objects of this invention include providing improved hydroxypropylgalactomannan compositions for use, e.g., in the manufacture of cosmetics, foodstuffs, drugs, paper, tobacco products or explosives or for use in printing or dyeing textiles, mining, or water treatment.